Cellulose Fiber Cores for Molding

The aim of this study was to manufacture high-strength articles using common box-making techniques, presumably of the paper setup box type whereby a low cost core or preform could be obtained with existing high speed machinery. The low cost core is then covered with a thin layer of high strength fibers, usually glass, the resin applied, and the whole preform placed in the mold. Using hard, stiff paper as the core, radii must be kept to a minimum. Greater radii and compound curve were possible when using a soft spongy core. With a stiff, heavy chipboard gentle compound curves were not too difficult. Blistering was a serious problem; therefore careful moisture control is very important. Most cores, especially the softer ones, are built up in layers and have little strength parallel to the layers. Threading or needling the core can prevent delamination. In some instances, holes punched in the soft core or hard cardboard are sufficient to avoid trouble. To avoid added weight and cost, a number of liquid sealants were tested to prevent resin impregnation. The best results have been obtained by using a barrier sheet of thin paper whenever the shape of the structure made this feasible. In some cases, both paper and liquid sealants were used. Fastening hardware is no problem where cardboard is used. As for the soft core, it is quite feasible to mold in a dense material in these areas to obtain good backing for rivets. Inserts of various kinds can also be molded in. In molding with a soft core, resin distribution is a serious problem. Complete spraying or dipping of the prepared preform has been found necessary to keep the resin content down and to insure even distribution of the resin. In some cases roller application might be used. Considerable difficulty has arisen on mold release. Some of the gases from the core given off under heat in the mold tend to contaminate the resin, making the cores stick. Also, the small amount of shrinkage of the part does not allow easy removal from the mold.

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